Among the most devastating diseases of crop plants, domestic animals, and humans are those carried by insects. Insects that transmit disease agents are called disease vectors.
The life history, host selection, and feeding behaviour of the vector largely determine the epidemiology of the disease. By controlling the insect, it is sometimes possible to break the link in transmission and to stop the spread of disease.
Microbes causing disease in plants and animals rarely also cause disease in the vector even when they live and multiply in the vector.
An exception is Western X mycoplasma, which reduces the longevity of its leaf hopper vector. Yet certain agents of plant disease actually seem to be beneficial to the vector by favourably changing the host plant’s structure or functioning.
Experiments have shown that vectors may feed or ovipost more frequently on diseased plants. A comparable relationship between animal diseases and their vectors is not known.
The nature of the transmission of pathogenic organisms by insect vectors to plant hosts may be mechanical or biological. Mechanical transmission involves the contamination of the body or mouthparts, or at most the foregut, after feeding on an infected host.
The pathogen is transferred to a new host when the vector next feeds or makes contact. Infectivity is lost when the supply of pathogen is depleted or at ecdysis if the vector is immature.
For example, mosquitoes function literally as flying pins in the transmission of the myxoma virus among rabbits. Similarly, the nonpersistent plant viruses are rapidly spread by aphids during their brief and repeated probing behaviour in search of suitable plant hosts.
Biologically transmitted pathogens, acquired after prolonged feeding, enter the vector’s gut, hemocoel, or various organs. Infectivity is not immediate, but occurs only after a latent period during which the pathogen moves about in the host or further develops or reproduces. Once infective, the vector tends to remain infective for life.
The persistent circulative plant viruses, such as the aphid-transmitted pea enation mosaic, do not replicate in the vector, but circulate in the body. On the other hand, helminths causing animal diseases undergo developmental changes but do not multiply in their intermediate intect hosts. This is called cyclodevelopmental transmission of animal pathogens.
When the pathogen multiplies in the vector, the transmission is called propagative. Persistent propagative plant pathogens, such as the aphid-transmitted lettuce necrotic yellows, and plague bacteria in fleas, are examples. If the multiplication is accompanied by cyclic development, as in the case of malaria Plasmodia in anopheline mosquitoes, the term is cyclopropagative.
Certain insects and acarines acquire infections as immatures and remain infective after one or more ecdyses. This is called transstadial transomission. Some transmit pathogens to their offspring by transvarial transmission, a feature already described above for endosymbionts.
Certain arboviruses (see below) and rickettsiae are transomission transmitted by ticks, but few insects have been shown to transmit animal pathogens similarly to their offspring. Plant pathogens, however, can be transovarially transmitted. Some persistent viruses, rickettsialike pathogens; and mycoplasmas are transmitted in this way by leaf hoppers.